The 9800 is a good card, but the problem isnt so much how much power it needs, its more what power your already using.
For instance i have a 650W PSU with 3 SATA HDS, a Bluray lightscribe DVDRAM, a Radeon X1950 XTX Crossfire and an AMD64 X2 5200 all running, i had to unplug my Digital freeview card because the power drain was high.
If you dont have alot of USB devices, harddrives or cd-drives you should be alright. The 3870 X2 could be a safe bet and a nice upgrade or if they are out yet, the 4300 or 4500 HD radeons, apparently they have extremely low PSU needs, running on a full load the card will only take around 25 Watts from your total PSU! which is impressivex